The present invention relates to a tappet for a switchable valve, and more particularly, to a hydraulically actuated piston and a clutch member having a bore and arranged to be actuated by the piston for switching the valve off in a first switching position and for activating the valve in a second switching position for charge cycle control.
Patent Document WO-93/18 284 describes a tappet wherein, during the valve lift and when the piston is acted upon by pressure, no clear position is fixed between the valve stem and bore in the clutch member. This may result in high wear from friction as the valve stem dips into this bore. Although, in the case of a correspondingly unfavorable alignment of the valve stem relative to the bore of the clutch member, the clutch member may also take along the valve, this will only be along a partial range of the height of the lift. Under the effect of the valve spring, the valve from that position strikes back into the valve seat in an undamped manner which causes very disturbing noises and additional wear.
It is an object of the present invention to provide a tappet having a clutch member held in at least one end position in a secure and defined manner with its bore or supporting surface always aligned in a functionally appropriate manner with the valve stem.
This object has been achieved according to the present invention by providing a locking element operatively associated with clutch member, so as to interact with a stem of the valve such that, in a defined cam path range, when the valve stem is dipped or moved into the bore of the clutch member, the locking element is unlocked.
One of the principal advantages achieved by the present invention is that the locking element locks the clutch member in an end position in which the valve takes up a first switching position. Only under the influence of a defined cam path range for moving the clutch member is this locking released so that the clutch member can be released by the valve stem.
When the piston is acted upon by pressure, the clutch member is advantageously first moved into an intermediate position, and then, in a second cam path range, with the valve stem now situated outside the bore of the clutch member, the second end position of the clutch member is achieved. As a result, the change from the first switching position into the second switching position of the valve is ensured in a manner which is low in noise and wear. The reverse movement or the return of the clutch member into the first end position takes place as soon as the piston is without pressure. The locking element has a simple construction and is actuated in a forced manner by the end of the valve stem as a function of the second cam path range. In a first switching position of the valve and as a function of the cam path range, the tappet is held by the hold-down device in a position separate from the cam, whereby the friction between the cam and the tappet is reduced.
The bearing part is suitable not only for receiving the piston, the clutch member and the pressure spring in a constructively favorable manner, but it can also easily be integrated into the tappet which is constructed in a cup-like manner with a jacket part and a cup bottom. The bore for the piston and a guiding section for the clutch member can be integrated in the cup of the tappet in a bearing section, so that the bearing section and the guiding section can be a housing made in one piece with the cup. On the side facing the cam, this cup is provided with an insert which comprises a receiving device for the adjusting plate. This insert, made, for example, of an iron-metallic material, is provided with the locking element. The locking element has a locking catch which interacts with the stop of the clutch member and which is formed by a flexible tongue in the carrier plate of the insert.
The locking of the clutch member may advantageously take place by meshing parts of the clutch member and its guiding path. The locking can then be released in a simple manner by the displacement of the two structural members relative to one another (e.g., lifting by the valve stem).
The switching reliability of such a tappet can be significantly improved again if the clutch member is fixed also in its other end position by an unlockable locking element. As a result, it may be ensured that the switching operation takes place in both directions, in each direction, in a precisely defined time period or during a defined angular position of the cam. The switching operation is, therefore, carried out in a reliable and defined manner within a short time period, and increased wear is thereby excluded.
The second locking device can advantageously also be unlocked by the valve stem so that all switching functions may take place without any constructive interventions on the camshaft. This permits the use of standard camshafts. The tappets according to the present invention require the same installation space as conventional tappets without any switching device. This additional locking device may advantageously be integrated into the clutch member and its guiding path so that additional constructional expenditures are avoided and a secure functioning with a small number of components is ensured.